Clarify default epoch and step values in ModelCheckpoint (#16651)

(cherry picked from commit 679f0db)

Author: carmocca

src/pytorch_lightning/callbacks/model_checkpoint.py

@@ -80,7 +80,8 @@ class ModelCheckpoint(Checkpoint):
                 ...     filename='{epoch}-{val_loss:.2f}-{other_metric:.2f}'
                 ... )
 
-            By default, filename is ``None`` and will be set to ``'{epoch}-{step}'``.
+            By default, filename is ``None`` and will be set to ``'{epoch}-{step}'``, where "epoch" and "step" match
+            the number of finished epoch and optimizer steps respectively.
         monitor: quantity to monitor. By default it is ``None`` which saves a checkpoint only for the last epoch.
         verbose: verbosity mode. Default: ``False``.
         save_last: When ``True``, saves an exact copy of the checkpoint to a file `last.ckpt` whenever a checkpoint